Way too many new novelists have succumbed to the siren's song of "new project inspiration." So many good first novels have jumped the tracks when the novelist took a detour to write a suddenly-arrived idea that showed more promise and felt like it would be more fun than what the writer was already writing.
It's not inspiration. It is exhaustion. It's boredom. By the time we are several hundred pages into a first draft or even a couple of drafts down the line, we cannot help but tire of the story we are living with night and day. So our brain gives us an opportunity to goof off under the guise of being an inspired by a notion worthy of a momentary diversion to get it down before it flees. Most often, it is not inspiration and it is not worthy of our novels. And when we get back to what we were writing it is cold.
What's the solution? When a new notion arrives, jot it down in a To-Do or reminder form and get back to your manuscript. If it is a good idea, it will still be good after you finish your current project.